Selectively amplifying loud-speaking telephone



AJa'n. l28, 1958 J. F. H'EK, JR

SELECTIVELY AMPLIFYING LOUD-*SPEAKING TELEPHONE Filed Nov. 12, 1954 United States Patent f- J 2,821,572 C@ Patented Jan. ,21s, 1958 Application'Noveniber 12,1954', Serial No. 468,3:8'1

4 Claims. (Cl. 179'i1) This invention relates t'o' a selectively-:amplifying loudspeaking telephone, and more particularly to ia: balancedjunction type of two-way loud-speaking telephone. Its object.` is to permit theuse of a vgreater amplifier gain than heretofore without thefoccurrence of howling.

vReference ismadetto-the pending Unitedy Statesv patent application of. Joseph Houdek, .'Ir., for an Automatic Ivntereomrn'unieation Telephone System', Serial No. 281,997,1filed April l2, 1952, which illustrates the -use of, loud-speaking telephones in an Yautomatic switching intereommunieating.telephone system. There, theam-V plifiers Aof va` s calling 'loudspeaking telephone "must `be activatedf atth'efend of `the settingoperations, yan'dthus before thef-desiredyconnection is-cornpl'etedl and an'=answ'er is.: obtained, ,so ,as tof enable the caller'tohear-the usual busy .tone i in case the connection 4-is withheld becausel the' ,called` line is busy. d-Therefore, thedull non-howlingi-am' plificationpermitted by -a balancedejunction. :arrange-- ment` cannottherebe realized, forfthey artiicial-'lineim-v pedance used must bev acompromise between fa' balance: with 4theimpedanceof. the calling. line alono andrea balance: with the Aimpedance presented by the Vfcalling.` line f-when connected.with a calledline in. answered condition. This impedance difference is.particularly.great in. an intercom-4 munication .system which employs-lines of'short-physieak length aikidlofv consequent. low. impedance.

According. tothe invention; the overall amplification: at v a v.calling loud-speaking .telephone is= keptlowl untilY the call.v answered, and is thereupon `automatically increased; j Y v value, thuspermitting-,the impedanceofltheyarti-1 ticial line to be-.adjusted for the best balanceunderthe answered' condition. Moreover, the condition ofpartial amplitca'tio'nis automatically restored when the receiver is `replaced onthe call'ednline, `and thepreferred-arrange rnentis' one in which the .alteration in amplification.: occurs only in the transmitting path,.so -as to Vleave the: receiving path ,fully active at all. times when -a callis being, made. `On a received call, full` amplification is. rendered effective` on answering.,-

V.The `foregoing kand other objectsand features of this invention` and. the manner of attaining" them; will: becom'e more apparent, Yand the invention itself. will'fbe b'es't understood, by` reterence toy the` following' descriptionl of embodiments of the invention taken in conjunction with4 the-.accompanying 4drawings `comprising Figs. l 'and' 2V wherein: Y

'Fig'. lfshows' ak first embodiment whereinthe invention. is applied' to 'a lon'dspealting' telephone for use ini the. local intercommunica'tion-system of the cited prior appli'-u cation; and

1=`ig.12,` showsv a seeondfembodiment-wherein' the'invenl tion is applied to a conventional two-wire reverse-battery automatic telephone system.

fFrst embodment.-Fg.. 1

Referring to Fig. l1, Mthe illustrated'switchboard -100, ordinary Y telephone t station "S1 and' loud-speaking station S2acorrespondrespectively t'o items A100', Sl, and S2 in the cited pending application.

Referring to .stationSLffor example, a call is made therefrom by'closin'g a` loop between the L and G wires of its"line"L1, and. then vtransmitting pulses over the S wirevto step the single? brush `318 of its individual selector SEL-1 at switchboard 1.00. A. metallic -connection is completed by brushi231'8 between the Ll Wires of the calling andc:calledflines.R The answering station closesits .loop/(abetweenfitsiband' G wires) to complete the talking connection.

Assume that a call is made from loud-speaking'station S2 to stationSl'. 'The c'all is controlled by equipment .2.89,.which.1is.theesignalling and. control `apparatus shown in detail,in Fig..2 Iofthe cited applica-tion. The loop is closed from wire L ofpline, L2, overtW-ire 287 .and circuit Z22, 223, to wire G; thesteppingV pulses arel transmitted over `the;;S,Wire;. .the `called "station `is signalled; and then 'the `#jwireris ,connected to the terminal-sf of the amplifiers. l

vWhen the call isransweredfat station slntheltalking connectiony is. completed. At .station-82, the line;L2,i's connectedv through condenser .,216 and` transformer 2&5 Ato balaneedjunction. 214. Signals from microphones'201 .a.re-y coupled through transformer ,209, -volume' control ,210, transmitting. amplifier 2121, and condenser'2112 to ljunction 2124. `,Receivedsignals Ithrough'v junction 1214 lare,coupled throughrvolume control-:21.7, amplifier 2&8, Eandtransformer 21.9',` to speakerfZflL -Signals from VtheA .transmitting amplifier divideY through junction .2154` between -artiticial line-213 and transformer `215. Feedback .signals between.'.microphoney 201and loud-speaker v202V arey cancelled i to the extent that g the impedancey of, artificial. line 213- matches kthe impedance presented ythrough :transformer `21.5. Condenser 281 ,added to lthe circuit.. helps.-limit the-bandfwidthito sim plifybalancing lIf' station S2..is;called,.answering eonnects;the`;L 'and `wires. of .line L2 .to wire 287 .and the terminals respectively to complete the .talking connection.

Y According toV -the invention, the lline voltage. appearing .between wiret287 and.wire.G at-stationjS2`isused to control the. grid .bias of thejsecond halffofthedual-triode transmitting amplifier `21.1 between.a.potentia'1 which cuts ott output and .arnormalvoperating potential. The -bias network added.to..the.. circuitcornprises v-cols 282 and 2183, and very. high-resistance resistors 284,*,285f-and'286. The slider of.y resistor ,286 may'be adjusted until .the desired bias appears fat ampliiier211 under eachzoffthe ztwo line conditions (answeredorunanswered).

When.. a-.call Amade from the .loud-speaking 'stationj S2 "to station S1,plate 'voltage is. suppliedv to" the Vamplifiers '211 ,and [218 after thesetting .of the. selectorvgbrush 1318 and signalling.; is completed. Withthe .selector vswitehed through .to fcon'nect .the calling and called lines, adrwith the called station S1 not yet ,answ,ered, the4 voltage be- -tween wires. 28.7. .and G.. at station ,S21A is.- suchV :that: transxmittinghamplier .211 isv biased to`-bloclcfanyoutput through condenser 212. After station S1 is answered and4 the transmitter, or other low-resistance circuit, is bridged between the L and G wires of line L1; the voltage be tween wires 287 and G at station S2 results in a bias at amplifier 211 which permits normal output.

If the called line is busy, wire L of line L2 is connected to a source of busy tone (not shown), instead of being connected to the brush 318' and line L1. The line voltage at station S2 results in a bias value which cuts off output from amplifier 211. Therefore, busy tone may be heard through amplifier 218 and speaker'202, without feedback through the transmitting path.

On a call made from station S1 to station S2, when station S2 answers the loop is already closed at station S1, and therefore the proper bias condition is obtained for normal output from amplifier 211.

On any call between stations S1 and S2, if station S1 disconnects first, the bias will be obtained at station S2 which prevents output from amplifier 211.

Since the amplifiers of station S2 are operative for full two-way amplification only when the line L2 is connected to a line on which the station is answered, artificial line 213 may be designed for best impedance match with a talking connection.

The voltage between wires 287 and G at station S2 varies with the length of the line, resulting in more gain from amplifier 211 on a long line than on a short line. Therefore the bias circuit also acts as an automatic gain control.

Second embodiment-Fig. 2

Fig. 2 illustrates an embodiment of the invention as applied to a conventional automatic telephone system, wherein the answering of the called station is indicated by a reverse-battery supervision.

A loud-speaking station S711 includes a microphone 601 operating into transmitting amplifier 611 and a loudspeaker 602 from receiving amplifier 618, the amplifiers being coupled to the line terminals through balanced junction 614, transformer 615, and condenser 616. A polar relay 623 controls operation of amplifiers 611.

Assume that the party at station S711 desires to call station S712. Closing switch S connects station S711 to line L711, closing a direct-current loop through polar relay 623, which does not operate at this time. A line finder LF on multiple M711 at the exchange is seized and connected to the calling line. Pulscs are transmitted by means of calling device CD, operating selector SEL, and a connector C on multiple M712, to establish a connection to the called line L712. In connector C, battery is supplied to the calling line L711 by line relay 691 and to the called line L712 by back-bridge relay 692.

Transmitting amplifier 611 is adjusted to prevent output before the call is answered, so that signals from microphone 601 do not reach junction 614. Amplifier 618 is operative so that signals received from the line, such as busy tone, may be heard from loud-speaker 602.

When the call is answered and the loop closed at station S712, back-bridge relay 692 operates and at its contacts closes a circuit for supervisory relay 690. At the contacts of relay 690 battery is reversed for the calling line L711. Polar` relay 623 operates, and at its contacts closes a circuit over path 684 to render amplifier 611 operative for normal transmission.

The connection is released when the calling station S711 disconnects.` If the called station S712 disconnects first, the relays 692, 690, and 623 release, and output is cutoff from amplifier 611.

The connection from back-bridge relay 692 to the brush terminals of connector C is of a polarity such that, if station S712 is a loud-speaking station similar to sta- "tion S711, when the call is answered at station S712 its polar relay will operate and permit amplifier output for two-way transmission. y

It may be seen that when a call is made from a loud- 'speakng station S711, the ampliers are operative for '4 full two-way transmission only when the loop is closed at the called station. Any connection at a called station is a talking connection. Therefore the artificial hne 613 may have an impedance which is the best match under talking conditions.

While I have described above the principles of. my invention in connection with specific apparatus, it 1s to be clearly understood that this description is made only by way of example and not as a limitation to the sc ope of my invention as set forth in the accompanying claims.

l claim:

1. In a switching system including telephone lines having telephone stations respectively thereon, each station including local switch means operable and restorable between a normal position and an in-use position and means responsive to its operation to the in-use position incidental either to the initiation of a call or to the answering of a call for closing a direct-current path across the associated line, thereby altering the normal impedance of the line to an in-use value, means responsive to the closing of a direct-current path at a station for establishing a flow of direct-current over the associated line, means for rendering the flow of direct current on any line variable between a connected value and an isolated value according to whether or not another line in its in-use condition is connected therewith, means including switchboard means for directly connecting any calling line to any idle called line and for signalling the station thereon, one said station being a loud-speaking station including a microphone and a loud-speaker connected to separate circuit branches, amplifying means in at least one said branch, a local artificial line for substantially balancing the effective in-use mpedance of the associated telephone line combined with the in-use impedance of another line to which such line is connected, a balanced junction at the loud-speaking sta tion interposed between the said lines thereat providing branch terminals which are coupled to both lines and which are conjugate when an impedance balance exists between the artificial line and associated telephone line, the microphone and loud-speaker circuit branches being connected to the said branch terminals, whereby undesired regenerative acoustic feedback frorn speaker to microphone is avoided under a balanced impedance condition in that signals from the microphone branch are then cancelled from appearance in the loud-speaker branch, a resistance device in the line at the loud-speaking station and a circuit path for supplying a control bias to the amplifying means according to the voltage drop across the resistance device, means for biasing the amplifying means to a condition of low amplification responsive to the voltage drop across the resistance device with said isolated value of direct-current ow on the associated line, whereby the undesired regenerative acoustic feedback which would occur if a higher amplification were used Iunder the then existing condition of impedance unbalance is prevented, and means respon,

sive to the voltage drop across the resistance device with the said connected value of direct-current flow on the associated line for biasing the amplifying means to a condition of increased amplification at the said amplifying means for conversation.

2. A switching system according to claim 1, wherein the said amplifying means includes a transmitting amplifier in the said microphone circuit branch and a receiving amplifier in the loud-speaker circuit branch, and the said means for biasing the amplifying means to a condition of low amplification comprises means for substantially disabling the transmitting amplifier while leaving the receiving amplifier fully operative.

3. In a switching system according to claim 1, wherein in the said amplifying means is so arranged and connectedl that its amplification varies indirectly with the value of the said control bias, wherefore it is greater for lines of highA resistance than for lines of low resistance.

4. In a switching'system including telephone lines have.`

telephone stations respectively thereon, each station including local switch means operable and restorable between a normal position and an in-use position and means responsive to its operation incidental either to the initiation of a call or to the answering of a call to establish an in-use current-ow condition thereover, means including switchboard means for connecting any calling line to any idle called line and for signalling the station thereon, one said station being a loud-speaking station including amplifying means, the value of the in-use current-dow condition depending on the resistance of the line, control means in the line at the loud-speaking station, and the said amplifying means being so arranged and connected to the control means that its amplification varies indirectly with the value of the in-use current-flow condition of the associated line, wherefore greater gain is obtained for lines of high resistance than for lines of low resistance.

References Cited in the le of this patent UNITED STATES PATENTS 2,369,351 Herrick Feb. 13, 1945 2,369,352 Herrick Feb. 13, 1945 2,370,388 Baird Feb. 27, 1945 2,545,477 Levy Mar. 20, 1951 2,623,949 Jeanlin Dec. 30, 1952 2,655,557 Stanbury Oct. 13, 1953 

